Whether catching a fly or a bullet, few materials can match the toughness of silk.
Story by Adam Summers - Illustrations by Sally J. Bensusan
In 1887 an obscure California medical journal published a frontier physician's observations on the remarkable properties of silk. The doctor, George Goodfellow, noted that silk scarves and handkerchiefs were impenetrable to the bullets with which the ne'er-do-wells of Tombstone, Arizona, were shooting one another. Since then, a good many engineers, biologists, biochemists, and physicists have spent time trying to unravel the mystery of silk's resilience. Much of the early research focused on the silk spun by silkworms for their cocoons (and used in the making of parachutes and pantaloons, as well as scarves and hankies) because these caterpillars can be easily farmed. Spider silk, however, turned out to be an even better material for warding off projectiles. Recent research has revealed the molecular as well as the genetic basis for the way different types of spider silk behave under stress.
Examine an orb weaver's web early in the morning, when it is still covered with dew. Radiating from the center are strands of frame silk, which form a grid that supports the network of capture-silk threads. Right away, you'll notice that the capture threads sag under the weight of the dewdrops, while the strands framing the web shrug off the added burden. The frame, or dragline, silk is stiff and strong but not at all sticky. By contrast, the fly-catching capture threads in the spiral of the web are made of viscid silk—a strong, stretchy thread covered with droplets of glue. Spiders produce other types of silk (to wrap prey, for example), but these two types have been the main focus of biomechanical research.
John Gosline and colleagues at the University of British Columbia have been investigating the differences between dragline and capture silk. In particular, they are interested in three properties: stiffness, strength, and toughness. Stiffness refers to how much the silk resists when pulled, while strength is a measure of how much force it takes to break a strand of a given diameter. Dragline silk is about as stiff as nylon thread and, pound for pound, stronger than steel cable. Capture thread, which stretches like rubber, is not stiff at all, but it still has nearly one-third the strength of steel. Though these qualities are impressive, some man-made materials can match silks in both stiffness and strength. The toughness of spider silk—its ability to withstand a sudden impact without breaking—is quite another matter, however, and one of special interest to the military and to industry
Some materials fracture when struck or yanked on; others give ground but don't break unless more energy is used. Kevlar, the fiber found in bulletproof vests and bicycle helmets, has less energy-absorbing capability than does either dragline or capture silk. Dragline silk is especially remarkable in this regard: when weights are dropped on this type of silk, it can absorb up to ten times more energy than Kevlar can. On impact, most of the kinetic energy dissipates as heat. From the point of view of a hungry spider, this is much better than storing it as elastic energy—which might simply catapult a prey item right back out of the web.
Industrial interest in producing artificial spider silk has led to the cloning of certain spider genes that specify the proteins, called fibroins, that make up the silk. Some of these cloned genes have been inserted into goats, whose milk now contains "harvestable" fibroins. Ultimately, many researchers hope to create synthetic, enhanced genes and to produce commercially viable quantities of fibroins to be spun into miracle fibers for everything from brake pads to surgical sutures. Genetic research has shown that the dragline fibroin molecule has two, quite different components: highly organized microcrystals (20-25 percent of each molecule) and amorphous, spaghetti-like tangles. The microcrystals form linkages between fibroin molecules. The amorphous tangles are dry, rigid, and glassy, giving the fiber its stiffness. Unlike glass, however, this material is not brittle. As the silk is pushed and pulled, the tangles straighten out, allowing the material to stretch without breaking. Dragline silk can be stretched by about 5 percent and still bounce back. Stretching it more than this permanently changes the configuration of the fibroins (primarily in the amorphous tangles), and the silk is unable to regain its shape. However, the thread doesn't actually rupture until it has been stretched by 30 percent of its resting length; Kevlar fibers snap when extended just 3 percent.
A molecule of capture fibroin is made of the same two components but in very different proportions: it consists almost entirely of tangles, with very few microcrystals. In addition, hydrated glue on the surface of capture silk keeps the strands moist, thus rendering them even less brittle than dragline silk. Rubbery and viscid, this silk stretches up to about three times its length when pulled on and takes a good deal of abuse before breaking—an important quality in a material responsible for entangling prey.
A simple change in the relative amounts of the two fibroin components, and the addition of a gluey coating, thus results in very different behavior. If fibroin can be engineered with just the right proportion of crystalline regions, manufacturers might be able to produce garments that are stretchy enough to be comfortable and fashionable, yet stiff enough to protect the underlying skin. Bulletproof T-shirts anyone?